The Empire at its greatest extent in 555 AD under Justinian the Great (its vassals in pink)
The Byzantine Empire, alternatively known as the Eastern Roman Empire, was the predominantly Greek-speaking eastern half continuation and remainder of the Roman Empire during Late Antiquity and the Middle Ages. Its capital city was Constantinople (modern-day Istanbul), originally founded as Byzantium. It survived the fragmentation and fall of the Western Roman Empire in the 5th century CE and continued to exist for an additional thousand years until it fell to the Ottoman Turks in 1453 CE. During most of its existence, the empire was the most powerful economic, cultural, and military force in Europe. Both "Byzantine Empire" and "Eastern Roman Empire" are terms created after the end of the realm; its citizens continued to refer to their empire as the Roman Empire , and to themselves as "Romans".
The borders of the Empire evolved significantly over its existence, as it went through several cycles of decline and recovery.
The final centuries of the Empire exhibited a general trend of decline. It struggled to recover during the 12th century, but was delivered a mortal blow during the Fourth Crusade, when Constantinople was sacked and the Empire dissolved and divided into competing Byzantine Greek and Latin realms. Despite the re-establishment of the Empire in 1261, Byzantium remained only one of several small rival states in the area for the final two centuries of its existence. Its remaining territories were progressively annexed by the Ottomans over the 15th century. The Fall of Constantinople to the Ottoman Empire in 1453 finally ended the Roman Empire.
Despite the occasionally derogatory use of the terms "Byzantine" and "Byzantinism", the Byzantine bureaucracy had a distinct ability for reconstituting itself in accordance with the Empire's situation. The elaborate system of titulature and precedence gave the court prestige and influence. Officials were arranged in strict order around the emperor, and depended upon the imperial will for their ranks. There were also actual administrative jobs, but authority could be vested in individuals rather than offices.
In the 8th and 9th centuries, civil service constituted the clearest path to aristocratic status, but, starting in the 9th century, the civil aristocracy was rival led by an aristocracy of nobility. According to some studies of Byzantine government, 11th-century politics were dominated by competition between the civil and the military aristocracy. During this period, Alexios I undertook important administrative reforms, including the creation of new courtly dignities and offices.